In an internal combustion engine, the more the compression ratio is raised, the more the thermal efficiency is improved, while the leaner the air-fuel ratio of the air-fuel mixture is made, the more the fuel consumption is improved. In this case, in a conventional spark ignition combustion internal combustion engine using a spark plug to cause part of an air-fuel mixture to ignite and using flame propagation of the ignited flame to make the remaining air-fuel mixture burn, if raising the compression ratio, knocking occurs, so the compression ratio cannot be raised.
Further, if making the air-fuel ratio of the air-fuel mixture leaner, the ignited flame can no longer be propagated, so the air-fuel ratio of the air-fuel mixture cannot be made leaner. As opposed to this, auto-ignition combustion is possible even if raising the compression ratio and is possible even if making the air-fuel ratio of the air-fuel mixture leaner, so if performing auto-ignition combustion, it is possible to improve the thermal efficiency and possible to improve the fuel consumption.
Further, in this auto-ignition combustion, the fuel diffused in the combustion chamber is made to burn inside the combustion chamber simultaneously at multiple points. If the diffused fuel is made to burn simultaneously in multiple points in this way, the combustion temperature becomes lower as a whole, so the formation of NOX is suppressed. Further, there is sufficient oxygen present around the fuel, so formation of unburned HC is also suppressed. In this way, auto-ignition combustion has many advantages, so has been looked at closely since the past. Numerous internal combustion engines designed to perform auto-ignition combustion are known (for example, see Japanese Patent Publication No. 2011-153562A).